A level maths help!

PLease help me with this question, I'm stuck on part b but it would also be helpful if you could anwer part i too :_
it's question 3b
https://pmt.physicsandmathstutor.com/download/Additional-Assessment-Material/Maths/A-level/Edexcel/Pure/10.%20Vectors%20QP.pdf

So for question 3) have you sketched it in 2D (you could forget about the k component for visualization)?
a) Simply add the two vectors to get D
b) If its a trapezium, there should be two paths to get to the same point D. So form the equation and solve for p.

ah right okay I understand, so for b we just equate the two paths?

OD must equal OD
How you get there is irrelevant.